The present invention relates generally to control systems for direct current motors, and more particularly, to a system for automatically controlling the field of a direct current shunt motor to control motor speed.
Direct current shunt motors are successfully used in such material handling devices as forklift trucks. It is desirable from a standpoint of efficiency and safety to provide a motor with an automatic field control which enables the motor to quickly reach the desired speed as determined by the operator and maintain that speed under varying load conditions, such as when the vehicle travels up and down inclines.
Automatic control systems for shunt motors have been developed which control field current in response to motor operating conditions. In U.S. Pat. No. 3,378,746, issued Apr. 16, 1968, a system is disclosed in which the field current is regulated in accordance with armature current. In U.S. Pat. No. 3,569,809, issued Mar. 9, 1971, the field current is controlled in accordance with the difference of armature voltage and a motor speed voltage.